Department of Astronomy
Colloquia Schedule for Fall 2007

Sept. 4th

David Charbonneau
  Harvard - Smithsonian Center for Astrophysics
"The Era of Comparative Exoplanetology"

When extrasolar planets are observed to transit their parent stars, we are granted
unprecedented access to their physical properties. It is only for these systems that we
are permitted direct estimates of the planetary masses and radii, which in turn provide
fundamental constraints on models of their physical structure. Furthermore, such planets
afford the opportunity to study their atmospheres without the need to spatially isolate
the light from the planet from that of the star. Recently, astronomers have taken a first glimpse
into the atmospheric chemistry and dynamics of these puzzling worlds. I will review the most

recent results, and then describe a new observatory that we are constructing that will
survey 2000 nearby M-dwarfs with a sensitivity to detect rocky planets orbiting within
their stellar habitable zones.

Sept. 11th

Angela Speck
  University of Missouri - Columbia
"The Nature of Stardust: Astromineralogy and Circumstellar Dust
Around Evolved Stars"

Intermediate-mass stars (0.8 - 8.0 solar masses) are major contributors of new elements to
interstellar space. These stars eventually evolve into asymptotic giant branch (AGB) stars.
During the AGB phase, these stars suffer intensive mass loss leading to the formation of
circumstellar shells of dust and neutral gas, including the new elements formed during the
star's life. Eventually the star runs out of material to lose, and the central core collapses
and heats up. Meanwhile the material around the star (the circumstellar shell) drifts away
from the star. Once the central star is hot enough to have significant ultraviolet (UV) emission
it will begin to ionize the surrounding medium, and it becomes a planetary nebula. The newly-
formed elements then become part of the interstellar medium, from which new stars and their
planets form.
     Using a combination of observing techniques (e.g. infrared (IR) spectroscopy, visible, IR and
and sub-mm imaging) and laboratory IR studies, together with theoretical considerations
(e.g. kinetics and thermodynamics of the dust-forming region; nucleosynthesis models and
changing stellar chemistries) and meteoritic evidence, we investigate the structure and evolution
of the circumstellar dust and its environment and how the evolution of AGB stars (in terms of
chemistry, mass-loss rates, dust shell dispersion and the change from benign AGB star to
ionized planetary nebulae), leads to changes in the dust composition and distribution.

Sept. 18th

Siang Peng Oh 
University of California, Santa Barbara (UCSB)
"New Views of the High-Redshift Universe"

I discuss theoretical perspectives on present observations of the high-redshift universe
in Lyman alpha emitters and QSO transmission spectra, emphasizing that our constraints
on the state of the intergalactic medium z > 6 are still very uncertain. I discuss Ly-alpha
radiative transfer effects which complicate the interpretation of high-redshift Ly-alpha emitters.
I then turn to prospects for detecting the IGM in 21cm emission with upcoming instruments,
and focus on the importance of developing novel statistical techniques for mining the data.
In particular, I discuss prospects for detecting HII regions both statistically and in imaging data.

Sept. 25th

Eugene Chiang 
University of California, Berkeley
"Problems and Prospects in Planet Formation"

Planets form in disks. Planetary properties result from myriad processes within disks, some
of which are chaotic. We pose and offer solutions to a variety of problems associated with
protoplanetary disks: (1) How do T Tauri disks dissipate? (2) As dust grains settle toward
disk midplanes, what are the maximum densities attainable? Are these densities large enough
for gravitational instability? (3) How do densely packed systems of protoplanets ("oligarchies")
relax into solar system-like (and extrasolar system-like) configurations? (4) Does Brownian
motion of planets within planetesimal disks interfere with resonance capture? (5) What governs
the distinct surface brightness profiles of debris disks? Application will be made to transitional
disks systems, including TW Hyd and GM Aur; the circumbinary ring of KH 15D; Neptune and
resonant Kuiper belt objects; and the debris disk encircling AU Microscopii.

Oct. 2nd

Yun Wang   University of Oklahoma
"Dark Side of the Universe"

The cause for the observed acceleration in the expansion of the universe is unknown,
and dubbed "dark energy" for convenience. Dark energy could be an unknown energy
component, or a modification of Einstein's general relativity. I will examine the most
promising methods for probing dark energy, and discuss recent results and future prospects.

Oct. 9th

Stephen E. Strom   National Optical Astronomical Observatory (NOAO)
"Transition Disks: A Possible Key to Understanding Planet Formation"

The unusual properties of transition objects (young stars with an optically thin inner
disk surrounded by an optically thick outer disk) suggest that significant disk evolution
has occurred in these systems. To explore the physical cause(s) for the transition
disk phenomenon, we examine these demographics, specifically their stellar accretion
rates and disk masses, and compare these parameters with those of accreting T Tauri stars
of comparable age. We find that transition objects of ages approximately 1 Myr occupy
a restricted region of the [mass accretion rate, disk mass] plane. Compared to accreting
T Tauri stars, transition disks have stellar accretion rates that are typically about 10 times
lower at the same disk mass, and disk masses about 4 times larger than the median
disk mass. These properties are anticipated by several proposed planet formation theories
and suggest that the formation of Jovian mass planets may play a significant role in
explaining the origin of many transition objects. We suggest observational strategies
that have the potential to determine (a) whether transition disks indeed indicate the onset
of planet formation; and (b) if so, how the physical characteristics of the transition disks
may be linked to outcome planetary system properties.

Oct. 10th

Special Event: Antoinette de Vaucouleurs Memorial Lecture
(Standard Location: RLM 15.216B, Time 3:30 p.m.)

John C. Mather   (Nobel Prize in Physics, 2006) NASA/Goddard Space Flight Center
"Finding our Origins with the James Webb Space Telescope"

How did we get here? Where are we headed? Dr. John Mather will tell the history of the universe
in a nutshell, and describe what our future holds within the realm of discovery. Dr. Mather is
Project Scientist for the James Webb Space Telescope (JWST), which is planned for launch
in 2013. As a successor to the Hubble Space Telescope, the Webb telescope will look even
farther back in time and examine the first stars and galaxies that were created after the big bang.
JWST will be the largest telescope mirror ever placed in space and with a positioning of
1.5 million miles away from Earth, the Webb telescope will be able to unravel some of the
biggest mysteries of the universe.

Oct. 11th

Special Event: Antoinette de Vaucouleurs Public Lecture
(Special Location: ACE 2.302 - Avaya Auditorium, Time 4:00 p.m.)

John C. Mather   (Nobel Prize in Physics, 2006) NASA/Goddard Space Flight Center
"From the Farm to the Nobel Prize: Deciphering the Big Bang"

The history of the universe in a nutshell, from the Big Bang to now, and on to the future -
John Mather will tell the story of how we got here, how the Universe began with a Big Bang,
how it could have produced an Earth where sentient beings can live, and how those beings
are discovering their history. Dr. Mather grew up on the Dairy Research Station in Sussex
County, New Jersey where he developed his strong interest in science. At Nasa, he was
Project Scientist for the Cosmic Background Explorer (COBE) satellite, which measured
the spectrum (the color) of the heat radiation from the Big Bang, discovered hot and cold spots
in that radiation, and hunted for the first objects that formed after the great explosion. He will
explain Einstein's biggest mistake, show how Edwin Hubble discovered the expansion of
the universe, how the COBE mission was built, and how the COBE data support the Big Bang
theory. He will also show NASA's plans for the next great telescope in space, the James Webb
Space Telescope. It will look even farther back in time than the Hubble Space Telescope, and
will look inside the dusty cocoons where stars and planets are being born today. Planned
for launch in 2013, it may lead to another Nobel Prize for some lucky observer.

Oct. 14 - 16

Frank N. Bash Symposium 2007   New Horizons in Astronomy
The Second Biennial Symposium on the Topic of New Horizons in Astronomy!
(Scientific Organizing Comittee: Kurtis Williams and Justyn Maund [co-chairs], Kyungjin Ahn, Eiichiro Komatsu, Mike Montgomery)

The Astronomy Program at the University of Texas at Austin is hosting its second biennial
symposium on the topic of New Horizons in Astronomy. This symposium brings truly excellent
young researchers who are working on frontier topics in astronomy and astrophysics together,
to exchange research ideas, experiences, and their visions for the future. The symposium
will focus on invited review talks given by postdoctoral fellows, followed by open panel
discussions, and a select number of poster papers from postdocs and graduate students
will be presented.

Oct. 23rd

John E. Chambers   Carnegie Institution of Washington / Department of Terrestrial Magnetism
"How Does Orbital Migration Affect the Oligarchic Growth of Planets?"

Many of the main characteristics of a planetary system are shaped during the oligarchic
growth stage of planetary formation. This begins when solid bodies in a protoplanetary disk
have grown to the size of large asteroids. In the Solar System, the end products of oligarchic
growth were Moon-to-Mars sized bodies in the terrestrial planet region. In the outer Solar System,
bodies grew substantially larger and were destined to become the cores of giant planets.
Current analytic theories and numerical simulations indicate that bodies formed during
oligarchic growth underwent rapid inward orbital migration caused by tidal interactions with
gas in the disk. In this talk, I will examine how planets might survive migration during
oligarchic growth, and look at the variety of planetary systems that can be produced as
a result of these processes.

Oct. 30th

Alicia M. Soderberg
  Princeton University
"A Radio View of the GRB-SN Connection"

Over the past few years, long duration gamma-ray bursts (GRBs), including the subclass of
X-ray flashes (XRFs), have been revealed to be a rare variety of Type Ibc supernova (SN Ibc).
While all these events result from the death of massive stars, the electromagnetic luminosities
of GRBs and XRFs exceed those of ordinary Type Ibc SNe by many orders of magnitude.
The observed diversity of stellar death corresponds to large variations in the energy, velocity,
and geometry of the explosion ejecta. Using multi-wavelength (radio, optical, X-ray) observations
of the nearest GRBs, XRFs, and SNe Ibc, I show that while GRBs and XRFs couple at
least ~10^48 erg to relativistic material, SNe Ibc typically couple less than 10^48 erg to their
fastest (albeit non-relativistic) outflows. Specifically, I find that less than 3% oof local SNe Ibc
show any evidence for relativistic ejecta which may be attributed to an associated GRB or XRF.
Recently, a new class of GRBs and XRFs has been revealed which are under-luminous in
comparison with the statistical sample of GRBs. Owing to their faint high-energy emission,
these sub-energetic bursts are only detectable nearby (z <~ 0.1) and are likely 10 times more
common than cosmological GRBs. In comparison with local SNe Ibc and typical GRBs/XRFs,
these explosions are intermediate in terms of both volumetric rate and energetics. Yet the
essential physical process that causes a dying star to produce a GRB, XRF, or sub-energetic
burst, and not just a SN, remains a crucial open question. Progress requires a detailed
understanding of ordinary SNe Ibc which will be facilitated with the launch of wide-field
optical surveys in the near future.

Nov. 6th

Barbara Ercolano   Harvard - Smithsonian Center for Astrophysics
"The Temperature Structure of HII regions and Star Forming Galaxies
Ionized by Multiple Spatially Distributed Sources: 3D Photoionization
Models with MOCASSIN"

Spatially resolved studies of star-forming regions show that the assumption of spherical
symmetry is not realistic in most cases, further complication is added by the gas being
ionized by multiple non-centrally located stars or star clusters.
Geometrical effects including spatial configuration of ionising sources affect the temperature
and ionization structure of these regions. I will present the results of our on-going theoretical
investigation of these effects which made use of 3D photoionisation mosels using the 3D
Monte Carlo photoionisation code MOCASSIN with various spatial configurations and
ionisation sources. In particular I will illustrate the behaviour of temperature fluctuations
within the ionised region as well as that of metallicity indicators based on strong-line
methods, that are often our only means to determine the metallicity of extragalactic
star-forming regions.

Nov. 13th

Karen M. Leighly
  University of Oklahoma
"The Influence of the AGN Spectral Energy Distribution on Broad-Line
Region Emission and Kinematics"

Quasars are the most luminous persistently emitting objects in the Universe. Powered by
accretion onto a supermassive black hole, many of their observable properties should be
determined by the fundamental parameters of accretion: the black hole mass and accretion
rate. The black hole mass and accretion rate should, in turn, determine the shape of
broad-band continuum emission arising from the accretion disk in the central engine.
That broad-band continuum is responsible for powering the strong, broad emission lines
that are prominent in optical and UV spectra, and are an identifying feature of AGN.
Thus, the line emission may be used as a probe of the fundamental intrinsic properties
of AGN, if we can break the code. I will present recent work investigating the role of the
spectral energy distribution in determining AGN broad-line properties.

Nov. 20th

Neal J. Evans 
University of Texas at Austin
"Star Formation: From Cores to Disks"

Observations with the Spitzer Space Telescope and complementary data at other
wavelengths have provided more complete samples of star-forming regions. These provide
constraints on theoretical models of the origin of the initial mass function and evolutionary
stages. The early stages of star formation include the separation of dense cores from the
background molecular cloud, the evolution before point source formation, the infall into the
central source, and the formation of the disk. These events are usually associated with
changes in the SED associated with the Class System. The large sample available from
the Cores to Disks (c2d) program provides good statistics on the numbers of objects in
various stages, and these can be used to estimate timescales. The evolution of the disk
to planetary systems is probed by studies of more evolved systems. We show that several
paths are possible in this evolution. Finally, the evolution of chemical state from molecular
cloud to planet-forming disk is revealed by infrared spectroscopy.

Nov. 27th

Nathan Smith 
University of California, Berkeley
"Precursors to Supernova Explosions and Extraordinary Deaths
of Very Massive Stars"

I will discuss some observations of a few recent and extraordinary supernova explosions.
Among these is SN2006gy, which radiated more luminous energy than any other supernova.
It may be our first observed case of a so-called "pair instability supernova", thought to mark
the deaths of the first stars in the very early Universe (although SN2006gy was relatively
nearby), and it appears to have suffered a violent precursor mass ejection just 5-10 years
before the SN. It was probably the most massive star ever seen to explode, and I will
mention connections to one of the most massive stars in our own Milky Way galaxy, called
Eta Carinae. SN2006gy is one of a class of supernovae that are plowing into very dense
circumstellar matter ejected by the star in the decade or so preceding the final SN. I will
discuss how this episodic pre-supernova mass loss, combined with some other recent
clues, is forcing us to revise some of our fundamental paradigms of massive star evolution.